Process for depositing an aluminum film on a substrate by thermal vaporization



P. J. CLOUGH ETAL PROCESS FOR DEPOSITING AN ALUMINUM FILM' ON A SUBSTRATE BY THERMAL VAPORIZATION Filed Dec. 31, 1949 Jan. 5, 1954 Power FIG.

FIG. 2

INVENTORS Phi/[p J. C/ouq/v ATTORNEY Patented Jan. 5, 1954 UNITED STATES PATENT OFFICE Paoosss Fort DEPDSITING AK LUMiN'UM' FILM ON A SUBSTRATE 'BY THERMAL VAPORIZATION 11 Claims.

This invention relates to coating and more particularly to the coating of various substrate materials With aluminum by vacuum evaporation and deposition of aluminum. This application is a continuation-in-part of the copending application of Earl E. Chadsey, J12, Philip J. Clough, and Philip Godley II, Serial No. 117,124, filed September 22, 1949.

A principal object of the present invention is to provide improved processes for evaporating molten aluminum-in a vacuum for the purpose of providing a coat of aluminum on a substrate positioned ina vacuum chamber.

Another object of the invention is to provide a process of the above type which permits high evaporation rates over long periods of operation of an aluminum-evaporating crucible.

Still another object of the invention is to provide a process of the above type which permits the use of a crucible of elemental carbon, this crucible being so treated as to have a high resistance to attack by molten aluminum.

Still another object of the invention is to provide for high aluminum evaporation rates by the prevention of the formation of aluminum carbide scum on the surface of molten aluminum being evaporated in a carbonaceous crucible.

Still another object of the invention is to pro-' vide an improved crucible for use in processes of the above type. I

Other objects of the invention will in part be obvious and will in part 'appearhereinafter'.

The invention accordingly comprises the product possessing the features, properties and the relation of components, and the process involving the several steps and the relation and the order of one or more of such steps with respect to each ofthe others which are exemplified in the fol lowing detailed disclosure, and the scope of the application of which will be indicated in the num, it is desirable to uses, relatively smal1'cru-"- cible so that radiant heat from the area of the moltenaluminum does not unduly raise the temperature of the substrate to be coated. It is also desirable to feed aluminum into the crucible during the coating operation so as to provide for long periods of operation. It is additionally advantageous to employ crucible materials which are relatively inexpensive and which have a relatively long life. Additionally, it is extremely desirable to have crucible materials which are not readily subject to heat shock so as not to unduly lengthen the warm up and cool-off periods.

It has been found that materials comprising elemental carbon, such as amorphous carbon, graphite, and particularly mixtures of amorphous carbon and graphite have excellent characteristics, such as cheapne-ss, machinability, and resistance to heat shock when used as crucibles for coating aluminum. However, molten aluminum, when raised to temperatures on the order of 12(l0 to 1300 C., rapidly attacks such crucibles with the formation of aluminum carbide. This aluminum carbide is soluble in molten aluminum, but the concentration thereof quickly approaches the saturation point. Since the surface of the molten aluminum on which the evaporation takes place is cooler than the main body of the molten aluminum, the aluminum carbide tends to come out of solution at this surface with the formation of small crystals of aluminum carbide. These crystals, being lighter than aluminum, float on the surface of the molten aluminum'in the form of a scum. This scum decreases the effective area from which the aluminum can be evaporated, thereby cutting down the evaporation rate. This defect is particularly noticeable in those cases where solid aluminum is fed to the molten bath during the course of evaporation. Since the solid aluminum is at a temperature considerably below the temperature of the bath, chilling of the surface occurs when the solid aluminum strikes the bath. This chilling rapidly increases the rate of scum formation. The cool metal, being heavier than the hot metal, sinks rapidly to the bottom of the crucible where it reacts with more elemental carbon to form additional aluminum carbide. Thus the feeding of aluminum to the molten pool of aluminum first causes the precipitation of aluminum carbide in theform of a scum and, additionally, forms more aluminum carbide which is dissolved in the melt. Thus the melt continuously remains saturated with aluminum carbide and more and more scum is formed as the solid aluminum is fed to the melt.

It has been found that all of the above-mentioned disadvantages may be overcome, and greatly increased evaporation rates and greatly lengthened crucible life may be achieved by providing, in addition to the aluminum in the crucible, a material which forms a carbide which is substantially insoluble in the molten aluminum at the evaporating temperatures. It has been found that the group Na and group Va metals are particularly advantageous from this standpoint. By the group IVa and group Va metals we mean those metals in groups IVa and Va shown on the Periodic Chart of the Atoms, W. F. Meggers, 1947 published by W. M. Welch Manufacturing Company. Generally, the effectiveness of these additional metals for preventing scum formation lies in the following order of decreasing effectiveness: hafnium, zirconium, titanium, tantalum, columbium and vanadium. Of these zirconium, because of its relative abundance, is preferred.

It has been found that these group IVa and group Va metals may be utilized in conjunction with the molten aluminum in an elemental carbon crucible in a, number of Ways. The metals can be added, before or during the evaporation, in the form of compounds which are reduced by the molten aluminum to the metal with the subsequent formation of the carbide, or they can be added in the form of the metal. Since these metals have relatively low vapor pressures at the temperatures employed for evaporating aluminum, they do not contaminate the aluminum coat on the substrate.

The group IVa and Va metals operate in several ways to prevent scum formation. When the metal, such as zirconium, is employed, it acts to form zirconium carbide from aluminum carbide in the melt. Since the zirconium carbide is relatively insoluble in molten aluminum, it is precipitated out of the solution. This zirconium carbide, being heavier than aluminum, sinks to the bottom of the melt and does not form a scum. Additionally, the zirconium forms a film of zirconium carbide at the interface between the molten aluminum body and the elemental carbon crucible wall. When this film is about 0.5 mm. thick it is substantially impermeable to molten aluminum and thereby prevents attack of the crucible wall by the aluminum.

Referring now to Fig. 1, one preferred form of apparatus utilizing the present invention is shown. This apparatus comprises a vacuumtight housing ill providing a vacuum chamber 52, this chamber being kept at a pressure in the micron range by means of a vacuum pumping system schematically indicated at l4. Within the chamber I2 there is provided a means for supporting a base material to be coated, this means being shown schematically as a, first spool l6 and a second spool i8 carrying therebetween the base material 20.

The means for vaporizing the metal comprises a metal-holding crucible 22 having a main body portion 2 5 in which the aluminum 26 is to be held in molten condition and heated to a temperature sufiicient to vaporize the aluminum at a high rate under the pressure existing in the vacuum chamber i2. Extending from the top of the main body portion 2d, there is provided a lip 28 which is preferably formed integrally with the body portion 24. For providing heat to the aluminum 2% and the metal-holding crucible 22, there is included a heating means, schematically indicated at 32. This heating means preferably comprises an induction coil having a portion 34 for heating the main body of the crucible and the aluminum carried thereby and a portion 35 for heating the lip 28 of the metal-holding crucible. A suitable power supply 38 is provided for furnishing a, high frequency current to the induction coil 34, 36. The induction coil as, 36 is preferably a water-cooled coil, such as a copper tubing, through the interior of which Water is adapted to be circulated. As can be seen from Fig. 1, the turns in portion 35 of the coil, adjacent the lip 28, are closer together than are the turns in portion 34 of the coil, adjacent the main body portion of the crucible.

For preventing radiation heat loss from the crucible 22, there is provided an outer crucible 40, preferably made of a refractory material which does not conduct electricity, and a refractory packing 42 between the metal-holding crucible 22 and the outer refractory crucible 40. Means for feeding aluminum to the crucible are also preferably provided but not illustrated. Such means may comprise a Wire-feeding'mechanism which feeds Wire to the crucible at a rate equal to the rate of evaporation of the aluminum. Additionally, the aluminum may be fed to the crucible in the form of pellets by the use of a pellet-feeding mechanism of the type described in the application of Earl E. Chadsey, Jr., Serial No. 130,453, filed December 1, 1949, and now Patent No. 2,635,579.

In a preferred form of the invention the metalholding crucible 22 is preferably formed of an elemental carbon, such as amorphous carbon, graphite, or a mixture of graphite and amorphous carbon of the type commercially available under the trade name Graphitar. The outer crucible co preferably comprises a refractory material, such as silica, while the refractory packing 42 preferably comprises zirconia or beryllia. The induction coil is preferably provided with a suitable source of high frequency current of approximately 9,000 to 150,000 cycles per second from a usual high frequency power source. When a Graphitar crucible 22 is employed for vaporizing aluminum, and a frequency of about 100,000 cycles is used, the skin depth of the induced current is approximately .2 inch at the temperature employed. The wall of the crucible is preferably made slightly thinner than .2 inch so that some of the induced current flows in the molten aluminum charge.

In the use of the present invention, as embodied in Figs. 1 and 2, a numberv of alternative methods may be employed. In one preferred embodiment of the process, a quantity of aluminum 26 is provided in the crucible 22 in the form of pellets or the like. Also included Within the initial charge is a number of pellets of zirconium metal. Chamber #2 is evacuated by the pump It to about .2 micron pressure, and power is supplied to the induction coil 35, 36. The amount of aluminum supplied to the crucible 22 is preferably enough to fill the crucible to within about a quarter of an inch from the top thereof. As the aluminum melts, due to the applied heat, the zirconium dissolves, forming a mixture of the two metals. Both the aluminum and the zirconium attack the inner surface of the body portion 25 of the crucible, the aluminum forming aluminum carbide, most of which is dissolved in the melt, and the zirconium forming zirconium carbide, in situ, on the inner surface of the crucible. As the zirconium carbide formation progresses, it forms a substantially impermeable layer or film 25 on the inner surface of the crumay be used. "In addition the'other group i and group 'Va metals, -hafniuzn, titanium; tan- .talum, columbium---and-vanadium may equally be piblefihis layenpreventing riurtherreactionxbetween the molten --;aluminum and ;the.-icrucible body 24. Excess zirconium inthealuminum-melt also acts to reduce: the Eformedalum-mum carbide to aluminum: with :the formation iofxzirconium. carbide which-sinks tothe bottomeofmthe crucible.

As can be seen from an examination of .Fig.

.. 2, the. molten aluminum in .the crucible :tends :to

aluminum tendingtto creep out of the :crucible is vaporized before it can progress beyondthe edge of the lip 28. Thissaspectoistheinvention is described in moredetailand is claimed in the above-identified application of Chadsey etaaln Serial No. 117,124.

The alcove description of the operation of the invention has been limited to one preferred form thereof, wherein zirconium metal is utilized as the material for forming the relatively insoluble carbide 255 which protects the inner surface oi the crucible. As mentioned previously, zirconia, or otherr-educiblecompounds of zirconium,

used, either inthaiorm ohthe inetal or intlie form of conipcunds-reducible by aluminum at the temperature-employed.

The variousmetals-or-compounds thereofisuch 'as zirconium titanium or 'the like, may be utilized in a number ofediiierentfashions. These metal compounds. may... be added as i a slurry to the interior oi the crucibleprior-to' the loading.oi the aluminum therein. ilnthiscase -the carbon of the crucible reduces some of the metal compound and forms the carbide directly. Equally, the metal may be deposited by vapor deposition procedures on the interior of the crucible. These vapor dep osition procedures include, among others, such processes as vacuum evaporation and deposition of the metal on the inner surface of the crucible, and vapor phase thermal dissociation or reduction on this inner surface. These vapor deposition procedures are particularly advantageous in those instances where the elemental carbon is relatively porous, such as is in the case of most graphites. In this case the preforming of the relatively impermeable inner film of the group Iva or group Va metal carbide prevents permeation of the molten aluminum into the interior of the graphite crucible, and thus prevents destructive interior formation of aluminum carbide.

In still other forms of the invention the metal may be melted in the crucible to form the protective carbide on the inner surface thereof. The metal, or compound thereof, may be added in the form of pellets, powder, or rods to the initial charge or to the melt during the course of evaporation of the aluminum. When only a small charge of the metal is added with the aluminum initially, it is preferred that the metal or its compound be added during the feeding of the aluminum to the melt. When relatively large quantities of the metal are added, either in the form of a carbide skin 25 on the crucible or as a part of the initial charge, it is sometimes not necessary to add any more of the metal during long periods of vaporization. For example, when about two grams of zirconium per square inch of inner crucible surface are added with the ini- .:tial:chargazitihaswbeenifcund that thel'esuhing carbideifilm 25 formed :onthe: interior ofitheJcr-ualuminum for well over five hours. operation.

-Theseexcellent results are achievedinispite of Hchilling'which occurs due to-feeding oi aluminun vto the crucible during this time.

As a general proposition it appears that, when "high evaporation :ratesare to be=employed from an unlined crucible, it is preferable touseffthe group 1V0. or. group Va metals rather than i the reducible compounds thereof 'in :the aluminum *melt. vThis requirement .is believed to lbei'due to the fact that the reactionbetween the'molten aluminum and-the elemental carbon is faster the compound such as zirconia. .Thereior'egthe charge in the crucible becomes-very rapid y saturated with aluminum carbide with :the rapid formation ofcum upon the feeding ofcoool aluminum thereto. Once.the scum .of' aluminum carbide is formed, it has been found much easier to eliminate the scum by the addition of pure metal, such as zirconium, rather than by'the addition of the compound zirconia. In all cases it is generally betterto prevent excess aluminum carbide formation than to eliminate the aluminum carbide formation after scum .has been created.

' While preferred forms of the invention have been'described above, itshould not be limited thereto. "Forexample the cruciblemay be wholly "formed of the carbide .of-one of the groupTVa and'Va metals. However, such crucibles are :relatively expensive, are notas easily formed .or metchined, and are more subject-to. heat shock than the elemental carbon'crucibles containing the inner' lining of the metal carbide.

"Whilethe invention has been described in connection with a preferred type of crucible and heating element for use therewith, it should be understood that the invention is by no means limited to the particular embodiment shown. The crucible may be of the type disclosed in the application of Noble E. Hamilton, Serial No. 117,444, filed September 23, 1949, and now abandoned. Equally, other types of crucibles and heating systems of the prior art may be employed. While a crucible is preferred for holding the aluminum, in those cases where a strong electromagnetic field is utilized to hold the molten aluminum in a globular shape on a flat surface, the support for the molten aluminum may comprise elemental carbon including a surface is contact with the aluminum which comprises a carbide of one of the group IVa or group Va metals.

Since certain changes may be made in the above product and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The process for the deposition of an aluminum film on a substrate by thermal evaporation of molten aluminum in a vacuum which comprises heating aluminum to a temperature sufficiently above its melting point to evaporate the aluminum, maintaining the molten aluminum, during evaporation, on an elemental carbon support, and maintaining at the interface between the support and the molten aluminum a layer of a carbide of a metal taken from the class consisting of titanium, zirconium, hafnium, vanadium, columbium and tantalum so as to prevent undue attack of the elemental carbon support by the molten aluminum, said carbide layer being formed by contacting said carbon support with a material comprising said carbide-forming metal while said support is maintained at a sufliciently high temperature to form the carbide layer.

.2. The process of claim 1 wherein said metal comprises zirconium.

3. The process of claim 1 wherein said metal comprises titanium.

4. The process of claim 1 wherein said metal comprises tantalum.

5. The process of claim 1 wherein said metal comprises columbium.

6. The process of claim 1 wherein said metal comprises vanadium.

7. The process of claim 1 wherein said carbide layer is on the order of 5 mm. thick.

8. The process of claim 1 wherein said carbide surface is formed on said support prior to feeding aluminum thereto.

9. The method of eliminating formation of aluminum carbide scum and protecting an elemental carbon support against attack by high temperature molten aluminum during the vacuum evaporation of aluminum from the support which comprises dissolving in the molten aluminum a second metal selected from the class con sisting of titanium, zirconium, hafnium, vanadium, columbium and tantalum, said aluminum containing said dissolved second metal being maintained at a sufiiciently high temperature to form said second metal carbide layer.

10. In a process for the deposition of an aluminum film on a substrate by thermal evaporation of molten aluminum in a vacuum wherein the aluminum is heated to a temperature sumciently above its melting point to evaporate the alumi num. the improvement which comprises forming a surface layer of a carbide of a second metal on an elemental carbon support by contacting said elemental carbon support with a material comprising said second metal and maintaining said material and said support at a sufiiciently high temperature to form said carbide layer, said second metal being selected from the class consisting of titanium, zirconium, hafnium, vanadium, columbium and tantalum, and maintaining the molten aluminum, during evaporation, on the carbide surface layer so as to prevent attack of the elemental carbon support by molten aluminum.

11. The process of claim 10 wherein said material comprises an axide of one of said second metals.

PHILIP J. CLOUGH. EARL E. CHADSEY, JR-

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,094,028 Stroup Sept. 28, 1937 2,153,786 Alexander et a1. Apr. 11, 1939 2,184,705 Willmore Dec. 26, 1939 2,205,854 Kroll June 25, 1940 2,362,147 Mondolfo Nov. 7, 1944 2,363,781 Ferguson Nov. 28, 1944 2,382,432 McManus et a1 Aug. 14, 1945 2,385,333 Clapp et al Sept. 25, 1945 2,450,851 Colbert et a1. Oct. 5, 1948 2,450,856 Colbert et a1. Oct. 5, 1948 2,465,545 Marsh Mar. 29, 1949 2,497,125 Hulme Feb. 14, 1950 2,548,897 Kroll Apr. 17, 1951 

1. THE PROCESS FOR THE DEPOSITION OF AN ALUMINUM FILM ON A SUBSTRATE BY THERMAL EVAPORATION OF MOLTEN ALUMINUM IN A VACUUM WHICH COMPRISES HEATING ALUMINUM TO A TEMPERATURE SUFFICIENTLY ABOVE ITS MELTING POINT TO EVAPORATE THE ALUMINUM, MAINTAINING AT THE INTERFACE BETWEEN DURING EVAPORATION, ON AN ELEMENTAL CARBON SUPPORT, AND MAINTAINING AT THE INTERFACE BETWEEN THE SUPPORT AND THE MOLTEN ALUMINUM A LAYER OF A CARBIDE OF A METAL TAKEN FROM THE CLASS CONSISTING OF TITANUM, ZIRCONIUM, HAFNIUM, VANADIUM, COLUMBIUM AND TANTALUM SO AS TO PREVENT UNDUE ATTACK OF THE ELEMENTAL CARBON SUPPORT BY THE MOLTEN ALUMINUM, SAID CARBIDE LAYER BEING FORMED BY CONTACTING SAID CARBON SUPPORT WITH A MATERIAL COMPRISING SAID CARBIDE-FORMING METAL WHILE SAID SUPPORT IS MAINTAINED AT A SUFFICIENTLY HIGH TEMPERATURE TO FORM THE CARBIDE LAYER. 